OPS-lasers are increasingly being used instead of solid-state lasers to provide high-power continuous-wave (CW) laser radiation. OPS-lasers can provide an output beam of very high-quality. The lasers may be operated in a fundamental mode or in an intra-cavity frequency-converted mode.
An OPS-laser employs as a gain-medium an epitaxially grown, multilayer semiconductor gain-structure including active layers spaced apart by pump-radiation absorbing layers. The gain-structure is surmounted on a mirror-structure and the gain-structure and mirror-structure are commonly referred to by practitioners of the art as an OPS-chip. The mirror structure may be formed from epitaxially grown semiconductor layers, dielectric layers, or a combination of metal and dielectric layers.
The gain-structure of the OPS-chip is optically pumped by radiation from an edge-emitting semiconductor laser (diode-laser) or an array thereof. The radiation may be focused directly from the diode-laser or array. The radiation may also be delivered from the diode-laser or array by an optical fiber or an optical fiber bundle, and then focused on the gain-structure. Although OPS-lasers have a relatively high optical-(pump-to-output) efficiency, absorbed pump-radiation that is not converted to laser radiation generates heat, and that heat is generated in a relatively small volume of material. Because of this, an efficient cooling arrangement is essential for an OPS-chip.
Typically, the OPS-chip is soldered onto diamond heat-spreader which in turn is soldered onto a copper heat-sink. The diamond heat-spreader assists the transfer of heat from the OPS-chip to the copper heat-sink by laterally spreading the heat away from the area of the chip in which the heat is generated (the pump-radiation spot or simply pump-spot). The heat-sink may be simply a massive, passive heat-sink, or may be an actively cooled heat-sink. An extensive description of OPS-lasers, including heat-sink arrangements with a diamond heat-spreader, is provided in U.S. Pat. No. 6,097,742, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference.
All else being equal, the thicker a diamond heat-spreader the more efficient is the heat transfer from the OPS-chip to the copper heat-sink. A problem limiting the thickness of a diamond heat-spreader, however, is a mismatch of coefficients of thermal expansion (CTEs) between the copper heat-sink, the diamond heat-spreader, and the OPS-chip. If the CTE-mismatch is not compensated, an OPS-chip can be become curved, buckled, or even detached from the diamond heat-spreader. This will cause a performance decrease, or even catastrophic failure, of the OPS-laser.
It has been found that for any OPS-chip there is a particular thickness of diamond heat-spreader that will provide an “effective CTE” of the combination of the diamond and copper which matches the CTE of that OPS-chip. That diamond-thickness, however, is less than would be an ideal thickness for optimum heat-spreading of the OPS-chip.
Assuming that the effective CTE is made such that detachment of the OPS-chip does not occur, a limiting factor in how much power can be extracted from an OPS-chip is a phenomenon known to practitioners of the art as “thermal roll-off”, which occurs when total pump-power (and heat generated) rises to a certain level. As the pump-power is increased from a threshold value, output power increases in a more or less linear relationship to the pump-power, until, at a certain level, power increase is rapidly reduced by thermally induced absorption in the semiconductor materials of the OPS-chip. At pump-powers beyond that level, output power decreases, eventually falling to zero. There is a need for a heat-sink arrangement which will provide more efficient heat transfer from an OPS-chip to a heat-sink, and thereby increase the pump-power level (and corresponding output power) at which the onset of thermal roll-off occurs.